There is a common belief that nothing can be known about the afterlife by living humans. This assumes that those who have died have no way of communicating with the living and that the living have no way of visiting after death realms. Both of these assumptions I believe are false.
There is a large body of literature written by those who claim to have communicated with people existing in "afterlife" realms. This information lies under the heading "spiritualism". A list of some worthwhile spiritualist books (mostly available as free ebooks) dealing with the afterlife can be found here.
Some- having paid cursory attention to spiritualism- make the claim that it is all fraudulent. This idea seems to be based on the reports of a small number of cases where individuals were shown to fake such things as table tipping and the like in the late 1800's and early 1900's. Like all fields containing a wide variety of people not everyone within it acts with integrity. But this doesn't mean all reports are fraudulent. In fact, there are a considerable number of serious-minded and believable accounts in existence which are worthy of the scrutiny of anyone truly interested in this topic.
The next source of human knowledge of the afterlife comes from people who have had a "near-death" experience. These vary widely in content though many have common themes. Some are incredibly detailed while others seem simpler, but most seem to have great significance to those who have experienced them. Although there are those rusted-on materialists who claim these experiences are all the work of the dying brain there are plenty of reasons to take these accounts seriously and to study them in detail.
The third category of human knowledge of the afterlife comes from mystics, those who have visited after death realms while in altered states of consciousness, and those who have experienced out-of-body experiences (OOBE's). Such people, some of who are serious-minded and well respected people (such as Robert A. Monroe), have written some quite detailed accounts of the non-physical worlds they have visited in altered states of consciousness, including interacting with those believed to be "dead" (discarnate).
Other sources of knowledge of the afterlife come from common people who have had personal experiences that lead them to believe they have connected with loved ones who have died. Most of these experiences are confirmations that their loved ones still exist and are happy where they are rather than detailed accounts of the afterlife, itself.
Of all these sources of knowledge the most detailed accounts come from spiritualist sources, particularly in the many accounts written in the earlier parts of the 20th century. Some of these accounts are incredibly detailed and are fascinating reading. For those interested in these accounts there is quite a good list of mostly free ebooks here.
I understand that there are some who base their ideas on the afterlife on what is written in the bible but my own understanding is that these scant accounts not only lack detail but are often factually wrong. For example, the widely promoted idea of an eternal hell for non-believers is false. While some will create a hell for themselves after death due to their own ignorance and unsavory character such a state is not permanent and help is always available for those who ask for it.
AJ Miller (Jesus) discusses love and the various spiritual dimensions.